What is Voyeurism manga?

Voyeurism is the sexual interest in or practice of spying on people engaged in intimate behaviors, such as undressing, sexual activity, or other actions usually considered to be of a private nature. The principal characteristic of voyeurism is that the voyeur does not normally interact directly with the subject of his/her interest, who is often unaware of being observed. The essence of voyeurism is the observing but may also involve the making of a secret photograph or video of the subject during an intimate activity. When the interest in a particular subject is obsessive, the behavior may be described as stalking. Source: Wikipedia